1. Field of Invention
The present invention relates to intra-oral methods and apparatus for optically imaging a structure and creating representative 3D models from the images.
2. Background
Determination of the surface contour of objects by non-contact optical methods has become increasingly important in many applications. A basic measurement principle behind collecting range data for these optical methods is triangulation. Triangulation techniques are based on elementary geometry. Given a triangle with the baseline of the triangle composed of two optical centers and the vertex of the triangle the target, the range from the target to the optical centers can be determined based on the optical center separation and the angle from the optical centers to the target.
Triangulation methods can be divided into passive and active. Passive triangulation (also known as stereo analysis) utilizes ambient light and both optical centers are cameras. Active triangulation uses only a single camera and in place of the other camera uses a source of controlled illumination (also known as structured light). Stereo analysis while conceptually simple is not widely used because of the difficulty in obtaining correspondence between camera images. Objects with well-defined edges and corners, such as blocks, may be rather easy to obtain correspondence, but objects with smoothly varying surfaces, such as skin or tooth surfaces, with no easily identifiable points to key on, present a significant challenge for the stereo analysis approach.
To overcome the correspondence issue, active triangulation, or structured light, methods project known patterns of light onto an object to infer its shape. The simplest structured light pattern is just a spot, typically produced by a laser. The geometry of the setup enables the calculation of the position of the surface on which the light spot falls by simple trigonometry. Other patterns such as a stripe, or 2-dimensional patterns such as a grid of dots can be used to decrease the required time to capture the image surface.
The surface position resolution of structured lighting methods is a direct function of the fineness of the light pattern used. The accuracy of active triangulation methods depends on the ability to locate the xe2x80x9ccenterxe2x80x9d of the imaged pattern at each image capture step. A variety of real-world situations can cause systematic errors to be introduced that affect the ability to accurately determine the true imaged pattern xe2x80x9ccenterxe2x80x9d. Curved surfaces, discontinuous surfaces, and surfaces of varying reflectance cause systematic distortions of the structured light pattern on the surface which can increase the uncertainty in measuring the position of the surface being scanned.
Additional measurement uncertainty is introduced if a laser is used as the light source to create the light pattern. Due to the coherence of laser light, reflections from the surface create a random interference pattern, known as laser speckle, throughout space and at the image sensor. The result is an imaged pattern with a noise component that affects the xe2x80x9ccenterxe2x80x9d determination, causing measurement errors even from a flat surface. The difficulty of determining the xe2x80x9ccenterxe2x80x9d of the pattern is further compounded if the surface that the pattern is projected upon is not opaque but translucent. This type of surface can result in the projected pattern xe2x80x9cbloomingxe2x80x9d at the illuminated surface because of the diffusion of light throughout the object. A tooth is an example of a translucent object that represents a challenging task from which to obtain a surface contour with active triangulation.
The dental and orthodontic field is one exemplary application for digitally generating 3D models of structures. In many dental applications, a working model of a patient""s teeth is needed that faithfully reproduces the patient""s teeth and other dental structures, including the jaw structure. Conventionally, a three-dimensional negative model of the teeth and other dental structures is created during an impression-taking session where one or more U-shaped trays are filled with a dental impression material. Impression materials include, among others, compositions based on alginates, polysulphides, silicones and vulcanizable polyether materials. The impression material is typically prepared by mixing a base component and a hardener or initiator or catalyst component. The impression tray containing the impression material, in its plastic state, is introduced into the mouth of the patient. To ensure a complete impression, an excessive amount of impression material is typically used. While the tray and impression material is held in place, the material cures, and after curing, the tray and material are removed from the mouth as a unit. The impression material is allowed to solidify and form an elastic composition, which is the negative mold after removal. The working model is obtained by filling this impression with a modeling material.
Dental patients typically experience discomfort when the dentist takes an impression of the patient""s teeth. The procedure can be even more uncomfortable for the patient if the impression materials run, slump or are otherwise expelled into the patient""s throat. Such situations can potentially cause a gag reflex reaction from the patient. In addition to patient discomfort, the impression process is time consuming. Additionally, the impression process can be error-prone. For example, when the impression material is not properly applied, the resulting working model may not accurately reflect features on the teeth. Moreover, the model can show air bubbles trapped during the impression taking session. Depending on the accuracy required, such working model may not be usable and additional dental impressions may need to be taken. Further, the mold and working model are fragile and can be easily damaged. The need to store the fragile models for future reference tends to become a logistical problem for a dental practice as the number of archived models accumulates.
Automated scanning techniques have been developed as alternatives to the mold casting procedure. Because these techniques can create a digital representation of the teeth, they provide the advantage of creating an xe2x80x9cimpressionxe2x80x9d that is immediately transmittable from the patient to a dental laboratory. The digital transmission potentially diminishes inconvenience for the patient and eliminates the risk of damage to the mold. For example, U.S. Pat. No. 6,050,821 discloses a method and apparatus for intraorally mapping the structure and topography of dental formations such as peridontium and teeth, both intact and prepared, for diagnosis and dental prosthetics and bridgework by using an ultrasonic scanning technique. As claimed therein, the method can provide details of orally situated dental formations thus enabling diagnosis and the preparation of precision moldings and fabrications that will provide greater comfort and longer wear to the dental patient. Also, as discussed therein, infra-red CAD/CAM techniques have been used to map impressions of oral structures and make single-tooth prosthetics.
Also, in certain applications such as restorative dentistry that is preformed on visible teeth, such as incisors, aesthetic considerations require that the prosthetic interface with the original tooth surface be underneath the gum (sub gingival) to eliminate the sight of the xe2x80x9cjoining linexe2x80x9d. In preparation for the prosthetic, the patient""s tooth must be shaped to create a ledge or margin beneath the gum line where the prosthetic will be sealed to the existing tooth. To prepare this surface, the dentist typically places a retraction cord between the tooth and gum. The retraction cord creates a working space that allows the dentist to machine the margin around the tooth of interest.
In order for the finished prosthetic to be correctly sized and properly seated on the prepared tooth, it is essential that the impression of the prepared tooth contain an accurate representation of the sub gingival margin. Improper resolution of the margin in the impression and the subsequent creation of the prosthetic from this impression can result in a poor seal along the margin of the prepared tooth and the prosthetic. A poor seal along the margin has the potential to expose the underlying tooth to decay and the subsequent loss of the toothxe2x80x94the very thing the prosthetic was suppose to prevent. Two methods are commonly used to accurately capture the margin during the impression process. The first method uses a retraction cord to hold the gum away from the tooth surface to allow the impression compound to flow underneath into the sub gingival region. The second method uses an impression material with low viscosity that under pressure is forced underneath the gums and thus captures the sub gingival margin.
In addition to obtaining sub gingival access for the impression material, the area of interest should be dry and clean (dry field) to obtain an accurate impression. A dry field is needed because typical impression compounds are hydrophobic and the presence of moisture when using a hydrophobic impression compound results in bubbles in the impression. The dry field is typically created by the dentist directing pressurized air across the prepared surface just prior to placing the impression tray in the patient""s mouth.
From a surface imaging perspective, human teeth consist of two primary components: enamel and dentin. The bulk of the tooth consists of semi-transparent dentin that is covered by a thin translucent layer of enamel that consists almost entirely of calcium salts in the form of large apatite crystals. These micro crystals form prisms or rods with 4-6 xcexcm transverse dimensions oriented normally to the tooth surface. The main dentin structural component is micrometer sized dentinal tubes, which radiate with an S-shaped curve from the pulp cavity toward the periphery. The crystalline nature of the enamel surface results in an optically anisotropic medium that results in double refraction or birefringence of the incident light pattern. Further, the translucent nature of the enamel results in a spreading or blooming of the incident structured light pattern as observed at the image sensor. Similar to the enamel, dentin also exhibits birefringence as well as having the dentinal tubes act as light pipesxe2x80x94further contributing to blooming. The observed color of a person""s tooth is primarily the result of the frequency selective absorption and reflection of the dentin material.
To minimize the effects of the optical properties of teeth during imaging, several commercial systems (Sirona Inc. Cerac System and Orametrix Inc. Suresmile System) have the user apply a coating to the area that is to be imaged to create an opaque surface. Typically, titanium dioxide is used because of its"" high index of refraction. Titanium dioxide is a white pigment that is commercially available in one of two crystalline forms: anatase or rutile and is widely used for providing brightness, whiteness, and opacity to such products as paints and coatings, plastics, paper, inks, fibers and food and cosmetics.
To achieve its"" optical properties, titanium dioxide particles must be created with an ideal particle size of 0.3-1 xcexcm. In powder form, titanium dioxide must be applied to a thickness of between 40 to 60 particles to achieve opacity on the tooth surface. This introduces an error into the true surface contour of the tooth that can vary from 12 xcexcm to 60 xcexcm. Since many dental procedures require surface accuracies of 25-50 xcexcm the use of titanium dioxide imposes severe and unrealistic constraints on the error budgets of the remaining parameters involved with making an accurate measurement of the teeth surface contours. Further, because titanium dioxide is a crystalline material, it exhibits optical anisotropy so it is important that the applied thickness be sufficient to create a truly opaque surface to eliminate birefringence effects. In addition, because titanium dioxide is an optically rough surface, it provides no reduction in speckle noise if coherent light is used for the illumination source.
Systems and methods for generating a three-dimensional (3D) model of a structure include coating the structure with a luminescent substance to enhance the image quality, the luminescent substance having an excitation range; and capturing one or more images of the structure through at least one image aperture each having a frequency sensitivity, wherein the frequency sensitivity of each image aperture is maximized for the luminescent material emission range.
For accurately determining the surface contour of a non-opaque object, the system provides a luminescent coating be applied to the surface of the object and then illuminated with a structured light pattern at a wavelength, xcex1, which corresponds to the excitation maxima of the luminescent compound. The incident light at xcex1 induces the luminescent compound to emit isotropic radiation at xcex2. The luminescent emission will only occur where the light pattern is incident on the surface. An optical filter is used to restrict the input to the image sensor to a narrow region around the luminescent compound""s emission wavelength, xcex2, and filters out the incident pattern light at xcex1.
Advantages of the system may include one or more of the following. The system minimizes pattern blooming effectxe2x80x94when a light pattern is projected onto a translucent object both diffuse reflection and diffuse transmission occur. The effect of the diffuse transmission is to spread the pattern light in all directions within the object. Since translucent objects typically will a have relatively low reflection coefficient ( less than 5%) the reflected surface pattern image intensity as seen by the image sensor will not be significantly larger than the diffuse transmitted light within the objectxe2x80x94a phenomena which has the effect of making the pattern appear larger. Conversely, using a luminescent coating results in an unattenuated signal directly from the surface and xe2x80x9cnoise signalsxe2x80x9d that are reduced  greater than 95% by the reflection coefficient of the object.
The system also eliminates speckle noisexe2x80x94due to the independent nature of the excitation and emission processes of luminescence, the emitted photons are incoherent and thus do not constructively/destructively interfere in an ordered manner. The system works with luminescence compounds with small molecular size to minimize coating errorsxe2x80x94luminescent compounds are available which allow hundreds of layers of material to be used yet still maintain sub-micron coating depths on the surface being measured. Moreover, the frequency shift of emitted luminescent light away from the incident pattern illumination frequency allows greater image sensor sensitivity and reduces the dynamic range requirements.
The system also provides a spray orifice to coat dental structure with substance to improve the imaging capability. Images of the dental structures are captured with sufficient resolution such that the acquired images can be processed into accurate 3D models of the imaged dental structures. The images and models would have application in dental diagnosis and for the specification and manufacture of dental working models, dental study models and dental prosthetics such as bridgeworks, crowns or other precision moldings and fabrications.
Further, the system provides automated intra-oral scanning of all the dental structures in the jaw through an optical aperture and combines the information available in the entire set of images to create and present an accurate 3D model of the scanned structures. The system allows intra-oral images of dental structures to be taken rapidly and with high resolution such that the acquired images can be processed into accurate 3D models of the imaged dental structures. The images and models can be used in dental diagnosis and used for the specification and manufacture of dental prosthetics such as bridgeworks, crowns or other precision moldings and fabrications. In addition, the system produces 3D models useful in the diagnosis and treatment planning process for dental malocclusions. The system-produced data representing a set of dental images and models can be transmitted electronically to support activity such as professional consultations or insurance provider reviews, and the images and models may be electronically archived for future reference.
The digital 3D model of patient""s teeth and other dental structures has advantages over a conventional cast physical model due to the following: 1) 3D model efficiently created in a single step with accuracy meeting or exceeding the conventional multiple step impression technique; 2) reduced storage costs; 3) immediate, labor-free retrieval and archiving; 4) no model breakage; 5) integrates directly into computer based analysis tools for diagnosis and treatment planning; 6) digital models backup; 7) e-mails to colleagues, dental specialists, insurance companies; 8) access to information from home, satellite office; 9) effective presentation tool; 10) no mess and dust; and 11) no wasted staff time.
The above and other features and advantages of the present invention will be apparent in the following detailed description of the preferred embodiments of the present invention when read in conjunction with the accompanying drawings in which corresponding parts are identified by the same reference symbol.